1. Field of the Invention
This invention relates to a solid polymer electrolyte fuel cell and a method for producing an electrode of the polymer electrolyte.
2. Discussion of the Background
Conventional solid polymer electrolyte fuel cells include a proton-conductive solid polymer electrolyte membrane. Solid polymer electrolyte fuel cells generate electromotive force by electrochemical reactions between fuel gas (for example, H2 gas) and oxidizer gas.
The solid polymer electrolyte fuel cells produce the following electrochemical reaction between H2 gas as the fuel gas and the oxidizer gas as the O2 gas at an anode side.2H2→4H++4e−  (1) 
After the electrochemical reaction, the resulting proton (H+) passes through the solid polymer electrolyte membrane. Then the solid polymer electrolyte fuel cell produces the next reaction at a cathode.4H++O2+4e−→2H2O  (2) 
Accordingly, the electrolyte fuel cell produces an electromotive force between the anode and cathodes of up to 1.23 V.
The schematic drawing of the conventional solid polymer electrolyte fuel cell is shown in FIG. 3. To smoothly and efficiently produce the above electrochemical reactions of the fuel cell shown in the reaction formulas 1 and 2, gas diffusion electrode 1b in FIG. 3 plays a very important role.
For the solid polymer electrolyte fuel cell to generate electric power, each of the fuel gas and the oxidizer gas needs to be supplied to surfaces of catalytic layers 1d disposed on the electrodes. At the cathode, water is generated on the surface of the catalytic layer 1d, as shown in the reaction formula (2). This water covers the surface of the catalytic layer 1d to inhibit the oxidizer gas from being supplied to the catalytic surface 1d. 
In the anode 1bb protons (H+) generated by the reaction as shown in the reaction formula (1) hydrate or take the water to the cathode 1ba through the solid polymer electrolyte membrane 1a. But the water in the anode 1bb is absorbed and stops short. Therefore the solid polymer electrolyte membrane 1a becomes dried out. To prevent the catalytic layer 1d from being dried, the fuel gas supplied to the catalytic layer 1d at the anode 1bb is generally humidified. But the excessive humidification by the fuel gas inhibits the fuel gas from being supplied to the catalytic surface 1d at the anode. To avoid the flooding due to water generated by the reactions and the water added by the humidified fuel gas, the electrode has been mixed with a water-repellent 1c, i.e., Polytetrafluoroethylene (PTFE) and so on. But the excessive addition of the water-repellent 1c to the electrodes 1b increases the electric resistance both in the bulk and on the surface of the electrodes 1b and decreases the gas permeability of the electrodes 1b. The excessive hydrophobicity of the electrodes 1b inhibits the humidification of the solid polymer electrolyte membrane 1a. Furthermore, the water absorbed in the solid polymer electrolyte membrane 1a is taken away by the fuel gas and the oxidizer gas provided to the catalyst layer 1d. Therefore the solid polymer electrolyte membrane 1a is dried out.
In order to improve the efficiency of the catalyst activity by three-dimensionally using the catalyst layer 1d, the electrolyte of the electrolyte membrane 1a has been dissolved with a catalyst in a solvent and impregnated with a gas diffusion layer wherein the gas can be diffused. In this process, the electrodes acquire proton-conductivity and even hydrophilicity. The electrode 1b as a gas diffusion type electrode, i.e., the electrode formed with the gas diffusion layer, needs to easily and equally distribute the fuel gas and oxidizer gas to the surface of the catalyst layer 1d. For this to occur, and to increase the porosity content, the gas permeability and gas diffusion coefficient needs to be increased.
However, the excessive provisions of fuel gas and oxidizer gas causes the electrolyte membrane 1a to be dried out, so that the proton-conductivity of the solid polymer electrolyte membrane 1a is reduced.
In addition, the conventional separators 1e of the fuel cell as shown in FIG. 3 generally are formed with concave portions and convex portions relative to the electrodes, in sectional shape, in order to output electric current and supply the fuel gas and the oxidizer gas to the electrodes 1bb and 1ba, respectively. Since the fuel gas and the oxidizer gas pass through the concave portions of the separators to be provided to the electrodes, but not at the convex portions, the fuel gas and the oxidizer gas are not equally diffused or distributed over the whole surface of each separator 1e. The electrodes 1b (the gas diffusion type electrodes) must diffuse the fuel gas or the oxidizer gas from the concave portions to the convex portions of the surface of the separators in order that the concentration of the fuel gas or the oxidizer gas is equally diffused in the surface of the catalyst layer 1d. For the above function, the electrodes 1b (the gas diffusion type electrodes) are made of a material having a large gas diffusion coefficient (porous material). But, the above material (porous electrode) tends to remove water from the electrolyte membrane 1a, to dry the solid polymer electrolyte membrane 1a. 
As described above, it is necessary for the gas diffusion layer of the fuel cell to be made with an appropriate balance between hydrophilicity and hydrophobicity, and an appropriate gas permeability.
To form the electrode having hydrophilicity, hydrophobicity and gas permeability, the conventional electrode is by from the following process. First, carbon black CB and PTFE formed in a paste with a dispersion medium are shaped in a sheet form. Then the sheet is baked to sinter the PTFE. Alternatively, the CB and PTFE are sometimes impregnated with carbon cloth or carbon paper, then the impregnated sheet is baked.
Though the structure or content of the above electrode is decided by a complex agglutination/dispersion mechanism which is changed by types, contents and mixing methods of the carbon blacks CB, PTFEs and dispersion mediums, the internal structure of the electrode cannot be regulated depending on the designer's intent.